1. Field of the Invention
The present invention relates to an ultrasonic transducer, and more particularly to an anti-EMI ultrasonic transducer.
2. Description of Related Art
An ultrasonic transducer is used in many apparatus, such as a reversing vehicle sensor. The ultrasonic transducer uses piezoelectric ceramic board, so the piezoelectric ceramic board starts to vibrate to generate the ultrasonic wave in the air when a controller of the reversing vehicle sensor outputs an alternating voltage signal with 40 HKz to the piezoelectric ceramic board. When the ultrasonic wave is reflected by an obstacle outside the vehicle, the piezoelectric ceramic board generates a detecting voltage after receiving the reflected ultrasonic wave. Therefore, the controller of the reversing vehicle sensor calculates a distance between the vehicle and the obstacle according to the current from the piezoelectric ceramic board. If the distance is determined in an unsafe range, the controller alarms the vehicle driver.
Since the reflected ultrasonic wave is weaker than the ultrasonic wave generated by the piezoelectric ceramic board, the piezoelectric ceramic board generates the small detecting voltage (about 2 mV to 5 mV). In fact, the controller does not calculate the exact distance according to the small detecting voltage, so a signal amplifier is required. In general, the signal amplifier having 500-1000 gain can be used in the reversing vehicle sensor. The signal amplifier is electronically connected between the piezoelectric ceramic board and the controller, the small detecting voltage is amplified and then output to the controller. However, the signal amplifier not only amplifies the detecting voltage, but also amplifier the noise. The noise may be generated by the piezoelectric ceramic board and wires between the piezoelectric ceramic board and the signal amplifier. Therefore, the controller determines the distance between the vehicle and the obstacle according the amplified detecting voltage with the noise, the distance is easily inaccurate.
With reference to FIGS. 10 and 11, a conventional ultrasonic transducer has a metal casing (60), a piezoelectric ceramic board (62), multiple signal wires (63) and an encapsulation (65). The metal casing (60) has an inside face, inside wall, a metal ring (64) and an opening (61). The metal ring (64) is mounted the inside wall of the metal casing (60). The piezoelectric ceramic board (62) is mounted inside the metal casing (60) and has two opposite first and second electrode faces. The first electrode face is mounted on the inside face of the metal casing (60) and the second electrode face faces to the opening (61). The signal wires (63) has a first signal wire (632) and a second signal wire (631). One end of the first signal wire (632) is soldered to a metal ring (64), so the first signal wire (632) is electronically connected to the metal casing (60) and the first electrode face of the piezoelectric ceramic board (62) through the metal ring (64). One end of the second wire is soldered to the second electrode face of the piezoelectric ceramic board (62). The encapsulation (65) is formed in the metal casing (60) to seal the metal ring (64), the piezoelectric ceramic board (62) and parts of the first and second signal wires (632)(631).
Based on the foregoing description, since the piezoelectric ceramic board (62) is only sealed in the encapsulation (65) and the encapsulation (65) is an electric insulation material, external EMI signals (66) enters inside the metal casing (60) from the opening (61) easily. The EMI signals (66) further interfere the piezoelectric ceramic board (62) to generate the detecting voltage. Therefore, the detecting voltage generated by the piezoelectric ceramic board can have the EMI noise. Therefore, the detecting voltage with EMI noise is amplified by the signal amplifier and the controller determines inaccurate distance and erroneous alarm.
To overcome the shortcomings, the present invention provides an anti-EMI ultrasonic transducer to mitigate or obviate the aforementioned problems of the conventional ultrasonic transducer.